"I have to have the numbers," says Inkinen, co-founder of the real estate website Trulia Inc. and a top-ranked amateur triathlete, as he wheels his $8,000 Orbea road bike to the starting line for a race in October.

Inkinen, who holds $52.6 million of Trulia shares after the company's initial public offering last year, soon charged up Mount Diablo in California amid a pack of 600 on a brisk autumn Sunday. A black band around the chest monitored his heart as it sped to 156 beats a minute. Devices in a wheel hub measured his power output at 377 watts and his average speed at 13.4 miles an hour.

The data fed wirelessly into a computer on the handlebars so he could later upload his results to the website Strava and compare them against those of hundreds of other riders.

The 37-year-old Inkinen has been recording more than two dozen such variables for years, including sleep, mood and caffeine intake, seeking patterns he can exploit to improve his results in sports and business. Inkinen is part of an expanding universe of self-quantifiers who collect megabytes of personal data seeking an edge much like the devotees of quantitative analysis who transformed Wall Street.

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There is a dark side even at some levels of amateur competition. Swimmers, runners, cyclists and triathletes have been penalized for using illegal performance-enhancing substances. In addition, one cyclist trying to post the fastest time on Strava for a route — laying claim to the virtual title "King of the Mountain" — crashed and died in California, according to a lawsuit against Strava.

Lance Armstrong, the world's most prominent confessed doping cheat, competed on closely held Strava Inc.'s website until Wednesday, based on recent updates to his Strava profile. He held more than 150 running-course records and King of the Mountain cycling rankings (known as KOMs) before they disappeared from his Strava page.

San Francisco-based Strava, which advertised on broadcasts of last year's Tour de France, treats Armstrong as "just another member of the community," says Chief Executive Officer Michael Horvath. Some Strava competitors urged banning Armstrong because of his doping.

Strava and rival sites such as MapMyRide, TrainingPeaks, Garmin Connect and dailymile tap into the psychological elements that make sports rewarding, says Ian Bogost, a game designer who teaches at Georgia Institute of Technology in Atlanta. They exploit people's competitive instincts to get them to ride their bikes faster, or work harder.

"What is the medium- to long-term consequence of a social environment where everything is attached to direct feedback and immediate reward?" Bogost says. "I don't know what the answer is, but I'm a bit afraid of it."

Sami Inkinen nears the finish line at the Mount Diablo Challenge near Danville, Calif. Inkinen numbers among the growing number of triathletes using sophisticated data measurement to improve fitness. (Bloomberg News photo by Noah Berger)

Amateur self-quants provide a ready market for electronic gadgets that make data-gathering easier. These include wristbands such as Larklife, Jawbone UP, Nike FuelBand and Fitbit Flex. For cyclists, Garmin Ltd. just introduced the $500 Edge 810 wireless, touch-screen, GPS-equipped bike computer, to collect data and provide navigation and weather forecasts. The devices enable anyone to gather, analyze and compare data in ways that were once available only to elite athletes.

For Sami Inkinen (pronounced SAH-me INK-in-en), these tools only complement a self-quantification effort that began more than a decade ago as hand-drawn notes on three-hole paper. He was earning a master's degree at the Stanford Graduate School of Business near Palo Alto, Calif. Raised on a farm in Finland, Inkinen says he came to love data while studying for a master's in engineering physics at Helsinki University of Technology.

"People like to talk about feelings and emotions, even when it relates to performance," Inkinen says. "More often than not, there are biological fundamentals underneath it. If you can understand and measure that, then you can make pretty objective decisions about how to improve it."

Inkinen has a scatter-plot graph showing correlation between mood and athletic performance. The day he rode up the 3,200-foot (975-meter) Mount Diablo, he rated his mood at 4 on a scale of 1 to 5, where 5 means "I'm ready to change the world," he says. He generated the highest wattage at the lowest peak heart rate in the race, according to Strava, and finished eighth.

In his 27-column spreadsheet, Inkinen also notes meditation time, happiness, productivity, caloric intake, number of push- ups and other fitness activities. He says the data give him a unique ability to control circumstances, have helped him finish an Ironman triathlon in less than nine hours and contributed to the success of Trulia.

He has also annoyed his wife of two years, Meredith Loring, she says. The couple met through an online dating service, where Loring's profile said she enjoyed running. She proved it on their second date, challenging him to a trail run that lasted more than three hours because neither would suggest stopping.

"As soon as we get home from a run, he is in his spreadsheet writing down all of the numbers," says Loring, 33, a strategy consultant for health and wellness companies. She agreed to be interviewed during a nine-mile trail run through towering redwood trees in Muir Woods near San Francisco, where the couple lives.

Loring uploads her bicycle rides to Strava. She holds dozens of "Queen of the Mountain" titles, mostly on climbs around San Francisco. She says she likes the virtual racing because the pressures associated with organized events, such as getting to the race start on time, cause her anxiety.

Online competition makes cyclists ride more and try harder, says Bryan Borgia, co-founder of Topwater Capital Partners in South Norwalk, Conn. Borgia, 36, became KOM last July on a segment south of Portsmouth, N.H., for example. He covered a 0.6-mile stretch of Route 1A in 66 seconds, beating hundreds of other riders.

"I can get myself to go pretty hard," Borgia says. "But if it's for Strava and I'm going out to beat someone, I'm probably going to go harder."

William Flint was killed in June 2010 after his bike hit a car while he was riding a Strava KOM challenge in Tilden Park in Orinda, Calif., near San Francisco, according to a wrongful- death lawsuit his family filed against Strava in state court in San Francisco. Flint was trying "to regain his title" after another cyclist posted a time on Strava breaking Flint's previous record for the ride, according to the lawsuit.

Strava failed to ensure that its challenges took place on safe courses and encouraged "dangerous behavior," the plaintiffs said in the complaint. "Cyclists are encouraged to 'tear it up' on the road," they said.

The company filed counterclaims alleging that when Flint became a website member in 2009 he electronically signed the site's terms and conditions, indemnifying Strava and waiving any claims arising from use of the site. Flint was riding "recklessly" over the posted speed limit on the wrong side of the road, Strava said in court papers, and the company isn't liable because the death resulted from Flint's own negligence.

The company's lawyers on Jan. 24 asked the court to dismiss its counterclaims without prejudice, meaning it can reinstitute them later. The court filings don't give a reason why Strava dropped the claims. A trial is scheduled for July 1.

After the Flint accident, Strava made it possible for users to designate courses that may be hazardous, according to the company's CEO, Horvath. Flagged routes — mostly descents — no longer feature leaderboards or KOMs. The ride Flint was taking is now in that category, according to the Strava website.

"It gives the community a tool to help determine what's safe and what's not," Horvath says. "Too steep, too much traffic or road construction. Some users have shut down segments because there were ducks and squirrels along the path." The company hasn't been named a defendant in any other suits, Horvath says.

Breathing hard, Jeff Howell, a data project manager, crests a popular 4.2-mile climb south of San Francisco and hits stop on his bike computer. Wearing Strava cycling jersey and shorts, he says he's pleased to post his second-fastest time on the route. Another cyclist zips past without a helmet.

"I don't see Strava inducing that kind of bad behavior," Howell says. "You have to take responsibility for yourself and know your limits."

At San Francisco-based Trulia, Inkinen's fascination with data permeates the corporate culture, says Pete Flint, co- founder and CEO. The company surveys its 534 employees, known as "Trulians," every three months using a "happiness index" that Inkinen created, Flint says.

There is lots of Inkinen lore at the company. Flint relates how in the early days the co-founders were Wednesday night regulars at a sushi restaurant. The order never changed: seaweed salad, sashimi, edamame and water.

"As soon as I started to mix things up, there was a revolt from Sami," Flint says. Inkinen didn't want a change in diet to affect the data he was monitoring on himself. For the same reason, Inkinen went through a spell of eating "the same thing for breakfast, lunch and dinner: a can of cold Progresso chicken soup," Flint says.

Inkinen's competitive drive once spawned a push-up competition with several Trulia colleagues known as "the Gun Show" (in fitness circles, "guns" is slang for biceps). Over 22 days last June. Inkinen's spreadsheet shows that he knocked out 100 push-ups on 20 of those days. When he managed just 50 one day, he added pull-ups to make up the difference. And on the single day when he did none, he rode his bike 105 miles. But that isn't the whole story, says his wife, Loring.

"One guy beat Sami one day, so he started doing push-ups in every area," she says. "Every hour, he would just drop and do push-ups wherever we were." That included a Safeway parking lot. "This is my husband," she says.

Inkinen is convinced his data holds important clues. As he was learning English, according to Ken Shuman, Trulia's communications chief, he would occasionally forget such simple words as "banana." Digging through his charts, he linked a lack of sleep to the forgetfulness, Shuman says. Inkinen confirms the story.

The data let him down after the Mount Diablo ride in California, however. Inkinen was using the race as a final warm- up for his sixth appearance in the Ironman World Championship six days later in Hawaii. Comparing his Mount Diablo data from October with results a year earlier, he was confident he would be able to complete the Ironman in less than 9 hours again.

"Going in, I was fitter and faster than ever," he says.

He had plenty of reasons for confidence. Two months earlier, he won the amateur title in an Ironman race in Sweden, finishing in just over 8 hours, 24 minutes and winning a Strava KOM for the bicycling portion. Then he and Loring took a seven- day cycling trip through the Alps. In September, Inkinen competed in a half-Ironman in Las Vegas, where the temperatures exceed 100 degrees on the course.

As the Hawaii Ironman began, Inkinen was the amateur favorite, according to the event's website. He was in first place among amateurs after the 2.4-mile swim and 112-mile bicycle stage, and he retained the lead through the first half of the 26.2-mile running portion. That is when his brain told him something that his data didn't.

"I just started feeling ridiculously tired," Inkinen says. He did what Ironman competitors typically resist at all costs. He stopped and dropped out of the race.

In endurance competition, the brain is the "central governor," according to Tim Noakes, a professor of exercise and sports science at South Africa's University of Cape Town. In his book, "Lore of Running," he writes that exhaustion reflects changes in brain commands to the muscles rather than changes in the muscles themselves.

Afterward, Inkinen reviewed his data in search of an explanation. He says it may lie in a viral infection he had between the events in Sweden and Las Vegas. While he was ill, his resting heart rate rose 20 percent to 46 beats a minute from 38, according to the spreadsheet.

During the race in Hawaii, "my body or brain or both refused to continue to work," Inkinen says. "It doesn't matter if the device on your wrist says you can go; your brain stops you. If you were able to override it, you would maybe do permanent damage or maybe kill yourself."

Inkinen's central motivation "is improvement, not winning," he says. And he cites another reason for dropping out. Eighteen days after the Ironman, Inkinen was set to compete in the 161-mile La Ruta de los Conquistadores mountain-bike race across Costa Rica.

"'I didn't want to end up in the hospital and miss that trip," he said. "I made a business decision."